While desired for their large bandwidth capabilities and small size, light-transmitting optical fibers are mechanically fragile, exhibiting low-strain fracture under tensile loading and degraded light transmission when bent. As a result, cable structures have been developed to mechanically protect the fibers, hence rendering fibers a realizable transmission medium.
A potential application for an optical cable is in ducts where space may be scarce. Such a cable must be capable of withstanding tensile loads applied when being pulled into a duct and bending stresses due to bends and turns in the ducts and manholes. One cable particularly suited for such an application is described in U.S. Pat. No. 4,078,853, issued to Kempf et al on Mar. 14, 1978.
In one embodiment, the Kempf et al cable comprises a core of optical ribbons surrounded by a loose-fitting plastic-extruded inner tubular jacket; a thick, compliant insulative layer of polypropylene twine; and a plastic-extruded outer jacket reinforced with primary strength members. In the Kempf et al cable, the strength members are embedded and encapsulated in the outer jacket to achieve tight coupling with the outer jacket. During cable manufacture, the insulative layer of polypropylene twine, onto which the strength members are wrapped prior to outer-jacket extrusion, retreats from the strength members under the pressure of the outer-jacket plastic extrudant, thus allowing encapsulation of the strength members by the outer jacket.
The Kempf et al cable has sufficient tensile strength to reliably protect the core of optical fibers under tensile loading and sufficient bending flexibility to ease cable handling prior to, during, and following duct installation. However, in certain situations, greater tensile loads are expected, especially where ducts are extremely congested, and/or when ducts have been found to have more bends than previously expected. In the Kempf et al cable, bending flexibility decreases when more strength members are added to the outer jacket for increased tensile strength. However, greater bending flexibility is desired at the same time as higher tensile strength to ease cable handling and installation.
Therefore, there is a need to design an improved optical communication cable which is capable of greater bending flexibility and greater tensile strength at the same time.
Desirably, such a cable is also designed to perform reliably under sustained tensile loads.